This invention relates to a hydraulic brake booster for use in a brake system having manual actuation to effect a primary brake application and hydraulic actuation to effect a secondary brake application to alleviate or at least attenuate a resultant of situations which may contribute to wheel skid, wheel spin or dynamic control of a vehicle.
In hydraulic brake boosters of the type disclosed in U.S. Pat. No. 4,441,319, a valve member is moved by an input force applied by an operator to a brake pedal to regulate the flow of pressurize fluid from a source to an operational chamber. The regulated pressurized fluid acts on and provides a force to move pistons in a master cylinder and effect a brake application. In order to move the valve member, the input force must first overcome the resistance of a spring that urges the valve member into engagement with a seat to seal a source of pressurized fluid. Subsequently, fluid pressure acts on the seal and as a result, the frictional force acting on the seal also opposes the movement of the control valve which must be overcome to initiate the actuation of the control valve and the flow of regulated pressurized fluid to effect a brake application.
An effect of frictional resistance relating to seals is addressed and reduced by the structure for a brake booster as disclosed in U.S. Pat. Nos. 4,281,585; 4,539,892; and 4,625,515. This type brake booster includes a housing with a first and second bores connected to a pressure chamber with a power piston located in the first bore and a control valve located in the second bore. The control valve is connected to an input member by a lever arrangement. An input force applied to the input member through a brake pedal moves the power piston and control valve to allow pressurized fluid to flow into the pressurize chamber and act on the power piston to correspondingly move a master cylinder to develop an operational force and effect a brake application. While this brake booster functions in an adequate manner the control valve, lever arrangement, input member and powers piston are all exposed to the pressurized fluid during a brake application.
It has been suggested to incorporated structure in a hydraulic brake booster of a brake system to perform additional features such as traction control, dynamic operational control and anti-skid control. Unfortunately without structural adaptation, the brake pedal mirrors the movement of the power piston in the operation of the hydraulic brake booster in functional performance relating to such additional features. In order for the brake pedal to remain during such additional functions, structure is disclosed in U.S. Pat. No. 6,203,119 to move a control valve seat while holding the input member in a stationary position to meter secondary operational pressurized fluid to effect a secondary brake application.
A primary object of the present invention is to provide a brake booster having a balanced control valve arrangement for regulating pressurized supply fluid to power a first piston which in turn pressurizes operational fluid in a master cylinder to effect braking in a first set of wheel brakes and to directly supply the regulated pressurized supply fluid to effect braking in a second set of wheel brakes in response to either an operator input or a secondary input requesting a brake application.
The brake system of the present invention discloses a brake booster that includes structure to functionally effect traction control, dynamic control and anti-skid control features. In more particular detail, the brake booster includes a first housing with a first bore separated from a second bore. The first bore retains a first or power piston and is connected to a first set of wheel brakes while the second bore retains a control valve arrangement. The control valve arrangement is connected to an input member for the power piston by a lever arrangement and is positioned in the second bore by a second housing to define a supply chamber and an operational chamber. The supply chamber is connected to receive pressurized supply fluid while the operational chamber is connected by a first passage to a power chamber located in the first bore, by a second passage to a second set of wheel brakes and by a third passage to a relief chamber. With control valve arrangement in a rest position, the operational chamber is connected to the relief chamber while the supply chamber is sealed to retain the pressurized supply fluid. The second housing has a bore that retains a secondary actuation piston that is aligned with the control valve arrangement. The secondary actuation piston is maintained in a rest position by resilient members associated with the control valve arrangement. In response to an operator input force being applied to the input member, the power piston moves in the first bore to interrupt communications between the first bore and the relief chamber while at the same time, the lever arrangement moves the control valve arrangement to initially close the communication between the operational chamber and the relief chamber. Thereafter, the lever arrangement moves the control valve arrangement to allow metered flow of pressurized supply fluid into the operational chamber as regulated pressurized supply fluid. The regulated supply fluid is simultaneously communicated to the first bore and to directly effect a brake application of the second set of wheel brakes. The regulated supply fluid communicated to the first bore acts on the power piston, to pressurize fluid in the first bore and develop operational fluid that is communicated to effect a brake application in the first set of wheel brakes. On termination of the operator input, the power piston and control valve arrangement return to a rest position. The control valve arrangement is also responsive to communication of a second supply of pressurized fluid supplied to a secondary chamber under the control of an Electronic Control Unit (ECU). The second supply of pressurized fluid acts on the secondary actuation piston which initially moves the control valve arrangement to close the operational chamber from the relief chamber and thereafter moves the control valve arrangement to meter pressurized supply fluid into the operational chamber to created a regulated pressurized supply fluid that is selectively supplied to the first bore to develop the operational fluid for the first set of wheel brakes and directly to the second set of wheel brakes to effect a brake application as a function of a second brake request derived by inputs supplied to the ECU. The connection or linkage of the lever arrangement and the control valve arrangement includes a collar/sleeve member which allows for independent movement by the secondary actuation piston without a force component being transmitted into the input member such that a brake pedal through which an operator input is applied remains in a stationary position.
An advantage of the hydraulic brake booster of the present invention resides in a separate development of pressurized fluid for effecting a brake application in a first set of wheel brakes by regulated pressurized supply fluid which directly effects a brake application in a second set of wheel brakes.
A further advantage of the invention resides in a structural relationship of components for a hydraulic brake booster whereby a second housing engages a first housing to hold a control valve arrangement in the first housing such that a balanced pressure is maintained across the control valve arrangement and in that an input member, lever arrangement and a portion of the control valve arrangement is located in a low pressure relief chamber.